1. Field of the Invention
The present invention relates to a magnetic oscillating device, and a magnetic sensor, a magnetic head, and a magnetic reproducing apparatus using the magnetic oscillating device.
2. Description of the Related Art
In recent years, a giant magnetoresistive (GMR) element has been commonly used in a magnetic head of a hard disk drive. The spin valve film has a structure in which a magnetization free layer (free layer), a nonmagnetic layer (spacer layer), a magnetization pinned layer (pinned layer), and an antiferromagnetic layer are stacked. In the spin valve film, the magnetization of the pinned layer in fixed by exchange bias with the antiferromagnetic layer, and only the magnetization of the free layer is rotated by an external field so as to change the relative angle between the directions of the magnetizations of the pinned layer and free layer, making it possible to provide a high magnetoresistive effect (MR ratio). Thus, the spin valve film can sensitively detect magnetic signals recorded in a magnetic recording medium.
However, increase in the density of magnetic recording medium has increasingly reduced the size of recording bits and thus the amount of leakage flux, that is, the amount of signal field, from recording bits. Also, so-called thermal noise has significantly reduced the signal-to-noise ratio (SNR). Under the circumstances, to develop a novel element that can sensitively detect a signal field in a high SNR even with a reduced field becomes a major technical challenge.
On the other hand, a spin-wave generator has been proposed which utilizes the motion of magnetization generated when a current is supplied perpendicularly to the film plane of a magnetic multilayer having a structure similar to that of the spin valve film (see Physical Review B. Volume 54, 9353 (1996)). The spin-wave generator includes a three-layer structure of a first ferromagnetic layer, a nonmagnetic layer, and a second ferromagnetic layer. The first ferromagnetic layer has magnetization pinned in a certain direction. The magnetization of the second magnetization layer can rotate freely. When a current is supplied perpendicularly to the film plane of the three-layer structure, electrons are spin-polarized when passing through the first ferromagnetic layer. The polarized current is then injected into the second ferromagnetic layer, where the spins of the electrons interact with the magnetization of the second ferromagnetic layer to excite a spin wave. A sensitive magnetic sensor based on this new principle can be developed by utilizing external-field dependency of a spin wave generated by such a spin-wave generator.
However, since the magnetization of the first ferromagnetic layer in the two ferromagnetic layers is pinned and only the second ferromagnetic layer contributes to the oscillation of a spin wave, the above conventional technique provides a low output. Further, with the conventional technique, the relative angle between the directions of the magnetizations of the first and second ferromagnetic layers vary every moment during magnetic oscillation. This induces magnetic dipole interaction between the two magnetizations, resulting in unavoidable energy loss. Therefore, the conventional technique disadvantageously has low energy efficiency.